This application is a Continuation Application of International Application No. PCT/JP02/07266 which was filed on Jul. 17, 2002 claiming the conventional priority of Japanese patent Application No. 2001-218045 filed on Jul. 18, 2001.
1. Field of the Invention
The present invention relates to an optical element having a multilayer film which minimally absorbs the light in the vacuum ultraviolet region and to an exposure apparatus provided with the optical element.
2. Description of the Related Art
The fluoride material has such an excellent optical characteristic that the material is transparent over a wide light wavelength range from the infrared to the vacuum ultraviolet region. In particular, almost all of oxide materials are opaque at wavelengths of not more than 180 nm, while a large number of fluoride materials are transparent. Therefore, the fluoride is necessary and indispensable especially for optical thin films and optical element materials to be used for the vacuum ultraviolet region.
In recent years, the high integration and the high densification are progressively advanced for the semiconductor integrated circuit. In order that the line width of the semiconductor integrated circuit is thinned and the pattern is made to be further minute, it is demanded to further improve the photolithography resolution of the reduction projection exposure apparatus for producing the semiconductor circuit. In order to improve the photolithography resolution of the reduction projection exposure apparatus, the wavelength of the exposure light source has been hitherto shortened to the g-ray, the i-ray (wavelength: 365 nm), and the KrF excimer laser (wavelength: 248 nm). It is inevitable that the wavelength will be progressively shortened to the ArF laser (wavelength: 193 nm) and the F2 laser (wavelength: 157 nm) in future. The fluoride, which is also transparent with respect to the exposure light beam having the shortened wavelength, is used for the optical element such as lenses and prisms and the optical thin film such as antireflection films and polarization films with which the surface of the optical element is coated.
As much as several tens of optical elements, which are directed to a variety of ways of use, are arranged between the laser light source and the wafer on which the semiconductor circuit is exposed in the reduction projection exposure apparatus as described above. The surfaces of the optical elements are coated with fluoride thin films depending on their purposes respectively. It is a matter of course that the materials for the optical elements themselves and the fluoride thin films absorb the light. Therefore, the amount of light, which finally arrives at the wafer surface, is considerably decreased. In order to improve the exposure performance and the productivity, it is necessary that the decrease in light amount is minimized as far as possible.
As a result of diligent researches and developments for many years, the defect and the content of impurity, which cause the light absorption, are suppressed as far as possible in relation to the optical element materials themselves. Owing to the advancement of the polishing technique, the scattering on the element surface is lowered as well. On the other hand, the optical thin film has been hitherto formed by means of various PVD methods including, for example, the vacuum deposition based on the resistance heating and the electron beam dissolution, the vacuum deposition combined with the ion assist, the ion plating, the sputtering, and the ion beam sputtering.
Fluorine is deficient as compared with the stoichiometric composition in the fluoride thin film produced by any film formation technique other than the resistance heating type vacuum deposition method. As a result, the absorption edge wavelength is shifted toward the long wavelength side as compared with the ideal crystal, and any absorption band also appears due to the defect and the impurity. Therefore, in such a fluoride thin film, the light absorption is increased in the vacuum ultraviolet region.
That is, in the case of the electron beam dissolution type vacuum deposition, the deposition material of fluoride is evaporated by using the electron radiation energy. However, the separation is caused by the energy between the fluorine atom and the metal atom in a certain proportion. Therefore, the fluorine deficiency occurs in the vapor deposition film. In the case of the ion assist vacuum deposition and the ion plating, the thin film surface, at which the growth is continued on the substrate, undergoes the ion beam radiation and the plasma radiation. As a result, the fluorine divergence, which is caused by the collision of charged particles on the growth film surface, also occurs in addition to the fluorine divergence caused on the side of the vapor deposition material. Further, in the case of the sputtering and the ion beam sputtering, when the fluoride target is subjected to the sputtering by means of the ion impact, only the light fluorine atoms are selectively subjected to the sputtering due to the selective sputtering phenomenon. As a result, the fluorine deficiency occurs on the target surface.
Therefore, in the case of the respective film formation processes other than the resistance heating type vacuum deposition method, any fluorine-based gas is introduced into the film formation vessel during the film formation in order to supplement the fluorine deficiency of the deposited film. However, it is complicated to control the reaction, and a film, which is based on the stoichiometric composition, is not necessarily obtained with ease.
On the contrary, in the case of the resistance heating type vacuum deposition method, fluoride crystal grains, which serve as the raw material, are placed on a vapor deposition boat made of a high melting point metal such as molybdenum, tungsten, and tantalum, and the vapor deposition boat is heated by applying the electric power to evaporate the raw material fluoride crystals. The fluoride, which is evaporated in accordance with the reversible physical change of the evaporation caused by the heating, does not cause the fluorine deficiency. The evaporated fluoride is allowed to fly onto a substrate having a relatively low temperature installed at a position opposed to the vapor deposition boat. The fluoride is deposited and solidified on the substrate while effecting the adsorption and the desorption, and thus the fluoride is grown as a thin film. The fluoride thin film, which is formed on the substrate as described above, hardly causes the fluorine deficiency.
As described above, the fluoride thin film, which is produced by the resistance heating type vacuum deposition method, is more excellent than the fluoride thin films produced by the other film formation techniques in that the fluorine deficiency is not caused and the fluoride thin film has the stoichiometric composition. On the other hand, the fluoride thin film has the columnar or prism-shaped structure. Therefore, the fluoride thin film has such a drawback that the film is porous and it has a large surface area, as compared with the single crystals and the bulk polycrystals as well as the films produced by the other film formation methods.
This drawback results from the fact that the substrate (fluoride optical element) has the low temperature. However, if the substrate is heated to a high temperature of several hundred degrees centigrade or more, the optical constant of the optical element itself is changed. Further, the shape of the lens surface, which has been strictly finished, is also changed, and the intended function as the optical element is consequently lost. Therefore, the substrate temperature is suppressed to be low.
If the substrate temperature is relatively high after the particles evaporated from the evaporation boat are allowed to fly onto the substrate surface to effect the adsorption, then the structure of the formed film is consequently dense, and the structural irregularity is scarcely caused as well, because the adsorbed particles receive the energy sufficient to make the movement on the surface to stable positions so that the surface becomes smooth. However, when the substrate temperature is relatively low, then it is impossible for the adsorbed particles to obtain the energy sufficient to make the movement on the surface, and the particles are immediately solidified after the adsorption. Therefore, the structure of the consequently formed film is a coarse columnar-shaped structure, and the structural irregularity is frequently caused as well.
Water and various hydrocarbons are adsorbed on the surface of the optical element taken out to the atmospheric air after the completion of the film formation. In the case of the single crystals and the bulk polycrystals, the mass of the adsorbed matter is reasonably decreased, because the surface area is small. However, the film, which is produced by the resistance heating type vacuum deposition method, has the columnar-shaped structure, the film is porous, and the film has a large surface area. Therefore, the amount of adsorption of water and hydrocarbons is far and away increased. Water and hydrocarbons absorb the vacuum ultraviolet light. Therefore, the transmittance is deteriorated in the vacuum ultraviolet region, because the amount of adsorbed matters is increased and the light absorption is increased as the surface area of the optical thin film for the vacuum ultraviolet light is increased.
Usually, the optical thin film is formed by alternately laminating high refractive index films and low refractive index films. As for the film materials preferred for the vacuum ultraviolet light, LaF3 is exemplified for the high refractive index film, and MgF2 is exemplified for the low refractive index film. The high refractive index film herein refers to the film composed of the material having a refractive index higher than the refractive index of the substrate. The low refractive index film herein refers to the film composed of the material having a refractive index lower than the refractive index of the substrate. The middle (intermediate) refractive index film refers to the film composed of the material having a refractive index between those of the high refractive index film and the low refractive index film. Until now, the applicant has made diligent researches about the reaction between the fluoride material and water. As a result, the applicant has found out the following fact. That is, water makes only the physical adsorption to MgF2, and it does not chemically react therewith. However, water makes not only the physical adsorption to LaF3 but also water causes the chemical reaction therewith to produce Laxe2x80x94OH and Laxe2x80x94O bonds on the surface of LaF3 exposed to the atmospheric air. That is, the surface of LaF3 exposed to the atmospheric air is hydroxylated and oxidized. In other words, the LaF3 film approximately has the stoichiometric composition of La:F=1:3 in relation to the entire film. However, the fluorine deficiency occurs in the vicinity of the surface exposed to the atmospheric air as a result of the chemical reaction with water in the atmospheric air to cause the oxidation and the hydroxylation. The vacuum ultraviolet light is absorbed by the oxidation area and the hydroxylation area, and the vacuum ultraviolet light is not transmitted therethrough. Therefore, as for the LaF3 film, the physically adsorbed water not only absorbs the vacuum ultraviolet light, but also the oxidation and hydroxylation areas, which are disposed on the surface exposed to the atmospheric air, absorb the vacuum ultraviolet light. Therefore, the optical loss is extremely large as compared with MgF2. That is, the optical loss of the optical thin film for the vacuum ultraviolet light composed of the alternate laminate of MgF2 and LaF3 is determined by the optical loss of LaF3 film.
As having been already discussed, the LaF3 film, which is obtained by the resistance heating vapor deposition, provides the porous columnar-shaped structure having the large surface area. Therefore, the amount of physically adsorbed water as well as the hydroxylation and oxidation areas is overwhelmingly increased as compared with the LaF3 of the single crystals and the bulk polycrystals as well as the LaF3 film produced by any other film formation method. This exactly means the fact that the amount of absorption of the vacuum ultraviolet light is increased.
The optical thin film, which includes the LaF3 film having the large light absorption as described above, has been carried on the exposure apparatus for the vacuum ultraviolet light. The respective surfaces of the optical elements are coated with the optical thin films. That is, the number of the coating surfaces is twice the number of the optical elements. Therefore, the light, which arrives at the wafer surface after passing through several tens of coatings of the optical thin films each having the large optical loss, has a light amount which is about several % of the original light amount of the light source and which is extremely low. In other words, the light absorption of the porous LaF3 film having the large surface area has determined the exposure characteristics.
The laser durability of the optical thin film as described above has not been so high as expected, due to the hydroxylation and oxidation areas existing on the LaF3 surface. Therefore, the frequency of the exchange of optical element parts has been inevitably increased. In other words, the LaF3 film, which is porous and which has the large surface area, has affected the throughput of the exposure apparatus.
An object of the present invention is to provide an optical element having a multilayer film which minimally absorbs light in the vacuum ultraviolet region. Another object of the present invention is to provide an exposure apparatus provided with the optical element.
According to a first aspect of the present invention, there is provided an optical element comprising an optical substrate which is formed of one of calcium fluoride, barium fluoride, and strontium fluoride; and a lanthanum fluoride film which is formed directly on the optical substrate. The crystal of the lanthanum fluoride film may be grown in C-axis orientation on the optical substrate. In order to reliably allow the lanthanum fluoride film to undergo the crystal growth in the C-axis orientation on the optical substrate, it is desirable that a plane (surface) of the optical substrate, on which the lanthanum fluoride film is formed, is a (111) plane or a plane inclined by an angle within xc2x130 degrees, more preferably within 15 degrees from the (111) plane.
According to a second aspect of the present invention, there is provided an optical element comprising an optical substrate; an underlayer which is formed of one of calcium fluoride, barium fluoride, and strontium fluoride; and a lanthanum fluoride film which is formed directly on the underlayer. When a plane of the underlayer, on which the lanthanum fluoride film is formed, is a (111) plane or a plane inclined by an angle within xc2x130 degrees from the (111) plane, the crystal of lanthanum fluoride film may be grown in C-axis orientation on the underlayer. Since the lanthanum fluoride film is dense and has a small surface area, it scarcely involves the oxidation and hydroxylation areas as well as the structural defect. Accordingly, it is possible to reduce the optical loss of the optical element in the vacuum ultraviolet region. Therefore, the optical elements according to the first and second aspects of the present invention are preferably usable as an optical element equipped with an antireflection film in the vacuum ultraviolet region.
According to a third aspect of the present invention, there is provided an exposure apparatus for exposing a substrate with an image of a pattern formed on a mask, the exposure apparatus comprising an illumination optical system which illuminates the mask with a vacuum ultraviolet light beam; and a projection optical system which includes the optical element according to the first or second aspect and which projects the image of the pattern onto the substrate. According to a fourth aspect of the present invention, there is provided an exposure apparatus for exposing a substrate with an image of a pattern formed on a mask, the exposure apparatus comprising an illumination optical system which includes the optical element according to the first or second aspect and which illuminates the mask with a vacuum ultraviolet light beam; and a projection optical system which projects the image of the pattern onto the substrate. According to the exposure apparatuses in accordance with the third and fourth aspects, the illumination optical system or the projection optical system includes the optical element according to the present invention. Therefore, it is possible to decrease the optical loss in the illumination optical system or the projection optical system with respect to the exposure light beam, especially with respect to the light beam in the vacuum ultraviolet region. Further, it is possible to efficiently introduce the exposure light beam onto the substrate. As a result, it is possible to improve the throughput of the exposure apparatus in which the operation of the exposure apparatus would be otherwise interrupted or restricted due to the maintenance including, for example, the cleaning and the exchange of the optical element disposed in the illumination optical system or the projection optical system.
According to a fifth aspect of the present invention, there is provided a method for producing an optical element, comprising preparing an optical substrate which is formed of one of calcium fluoride, barium fluoride, and strontium fluoride and which plane is a (111) plane or a plane inclined by an angle within xc2x130 degrees from the (111) plane; and forming a lanthanum fluoride film on the plane of the optical substrate. According to a sixth aspect of the present invention, there is provided a method for producing an optical element, comprising forming an underlayer composed of one of calcium fluoride, barium fluoride, and strontium fluoride on an optical substrate while heating the optical substrate so that a (111) plane or a plane inclined by an angle within xc2x130 degrees from the (111) plane appears; and forming a lanthanum fluoride film on the underlayer. According to the production methods in accordance with the fifth and the sixth aspects, the lanthanum fluoride film undergoes crystal growth while being subjected to C-axis orientation from the (111) plane or the plane inclined by the angle within xc2x130 degrees from the (111) plane of calcium fluoride, barium fluoride, or strontium fluoride. Accordingly, it is possible to obtain the lanthanum fluoride film which is dense, which has a small surface area, and which scarcely involves the oxidation and hydroxylation areas as well as the structural defect. Therefore, it is possible to obtain the optical element in which the optical loss is reduced in the vacuum ultraviolet region.